1) Field of the Invention
The present invention relates to a drive force transmission mechanism for transmitting a drive force to a driven element. In particular, the drive force transmission mechanism according to the present invention can be advantageously applied to a printer to transmit a drive force to a driven element such as a platen, a paper feeder, and an ink ribbon cassette, etc., included in the printer.
2) Description of the Related Art
A serial printer, for example, a wire dot printer, comprises a printer frame, a platen rotatably supported by side walls of the printer frame, a carriage slidably mounted on a guide bar supported by the side walls of the printer frame, so that the carriage is reciprocatedly movable along a longitudinal axis of the platen, a wire dot printing head mounted on the carriage and having a printing face facing and close to a surface of the platen, and a paper feeder such as a pin-belt tractor provided between the side walls of the printer frame for feeding a continuous form paper to a printing position defined between the platen and the printing head. In this printer, the platen and the paper feeder are driven by a common electric motor mounted on one of the side walls of the printer frame, and a rotational drive force is transmitted from the motor to the platen and the paper feeder through the intermediary of a drive force transmission mechanism formed as a reduction gear train. This reduction gear train is provided on the side wall of the printer frame on which the motor is mounted, and is disposed between an output gear of the motor and two gears which are fixed on drive shaft ends of the platen and the paper feeder extended through the side wall of the printer frame.
Conventionally, each of the gears forming the reduction gear train is rotatably mounted on a shaft member projected from an outer wall surface of the side wall of the printer frame. When the printer frame is formed of a metal material, for example, steel, the shaft members are also formed of steel and are securely riveted to the side wall of the metal frame. On the other hand, when the printer frame is moulded from a suitable synthetic resin material, the shaft members are integrally moulded with the printer frame. In both cases, the shaft members are formed as a part of the printer frame and are permanently secured thereto, and accordingly, if one of the shaft members is broken, the printer frame per se must be replaced by a new unit. As is well known, the replacing of the printer frame is time-consuming and expensive, because the parts of the printer must be first disassembled and then reassembled.
In the prior art printer as described above, the carriage on which the printing head is mounted is provided with an ink ribbon cassette holder and an ink ribbon winder associated therewith. In general, an ink ribbon cassette has two reels, i.e., a driven reel and a drive reel, and the driven reel is wound with an ink ribbon having a free end thereof connected to the drive reel. The ink ribbon cassette is held by the ink ribbon cassette holder in such a manner that an ink ribbon of the cassette runs between the platen and the printing face of the printing head, and the driven reel of the ink ribbon cassette is engaged with the ink ribbon winder. During a printing operation, the ink ribbon winder is driven by a drive mechanism for the carriage, whereby the ink ribbon can be drawn from the driven reel and wound up by the drive reel. The ink ribbon winder includes a drive force transmission mechanism for transmitting a drive force from the carriage drive mechanism to the drive reel of the ink ribbon cassette, and the drive force transmission mechanism is formed as a gear train provided on the carriage. Each of the gears forming the gear train is rotatably supported on a shaft member projected from a body or main frame of the carriage, and these shaft members are also permanently secured to the main frame of the carriage. Accordingly, if one of the shaft members is broken, the carriage body or main frame per se must be replaced by a new unit. This replacing of the carriage body is also time-consuming and expensive, because the parts mounted on the carriage body must be first disassembled and reassembled.